Renin-aldosterone profiling was used to classify patients with hypertension: a) 243 patients with essential hypertension were classified by renin-urinary sodium indexing, b) 107 were reclassified by response to administration of furosemide and intravenous saline, c) 45 were further classified by response to a low sodium diet. Arbitrary "normal ranges" were determined in 89, 32, and 38 volunteers, respectively. The possibility is considered that low renin patients have a primary renal abnormality in renin secretion with relative deficiencies of angiotensin II and aldosterone when they are subjected to diuresis. They can maintain aldosterone secretion under normal conditions because their adrenal aldosterone receptor is supersensitive to angiotensin II. No evidence of abnormal sympathetic neutral activity was found among the renin subgroups. Renin-aldosterone profiling in clinical practice is useful mainly in the detection of patients with curable forms of secondary hypertension. Aldosterone/renin ratios obtained while a patient's extracellular fluid volume is expanded and contracted may be diagnostically helpful. BIBLIOGRAPHIC REFERENCES: Mitchell, J. R., Taylor, A. A., Pool, J. L., Lake, C. R., Rollins, D. E., and Bartter, F. C.: NIH Combined Clinical Staff Conference: Renin-aldosterone Profiling in Hypertension. Ann. Intern. Med. 1977 (In press).